As shown in FIGS. 1 and 2, an immersion photolithography apparatus is indicated generally by the reference numeral 100. The apparatus 100 may include a wafer stage 12, a wafer 14 disposed above the stage, a photo-resist (“PR”) layer 16 disposed above the wafer, a fluid medium 18 disposed above the photo-resist, a lens 20 disposed above the medium, a lens module 22 disposed above the lens, and a light source 24 disposed above the lens module. The fluid medium 18 contacts both the lens 20 and the photo-resist 16. The apparatus 100 may further include a fluid supply tank 26 for supplying fluid to the fluid medium 18, and a fluid recovery tank 28 for recovering fluid from the fluid medium 18. The fluid supply tank contains and supplies a supply liquid 27, which is a substantially purified liquid having a refractive index n. The fluid recovery tank 28 recovers and contains a recovery liquid or suspension 29, which is the liquid plus additives or contaminants having a refractive index n′.
Here, the resolution R of the optical system is defined by:R=Λ/NA  Eqn. 1NA=n sin Θ  Eqn. 2
In Equations 1 and 2, Λ is the wavelength of the main incident light, NA is the numerical aperture, n is the refractive index of the fluid medium between the lens module and the wafer, and Θ is the refracted angle between a central vertical axis of the lens module and the light going towards the focal point of the lens module from its edge region.
The NA is approximately in proportion to a diameter of the lens module and is approximately in inverse proportion to a focal distance d of the lens module. The resolution may be improved by increasing the refracted angle Θ or the refractive index n of the medium.
Conventional designs have required a design modification of the lens module to increase the refracted angle Θ for finer resolution. It has been proposed that the immersion photolithography apparatus may be supplied with an immersion liquid having a refractive index larger than that of air or vacuum between the lens module and the wafer. Unfortunately, because the immersion fluid directly contacts a photo-resist “photo-resist” layer of the wafer, an immersion liquid, particularly at the liquid to wafer boundary, may be contaminated by a photo acid generator (“PAG”) of the photo-resist. Thus, if some PAG contaminates the immersion liquid, the refractive index may be undesirably varied by the PAG, and the lens and/or lens module may be undesirably corroded by the PAG.
In addition, the refractive index of the immersion liquid varies according to the temperature of the liquid. For example, as the temperature of the immersion liquid rises, the density of the immersion liquid decreases, and, in turn, the refractive index of the immersion liquid also decreases. In cases where the refractive index of the immersion liquid varies for any reason, it may prevent the formation of a uniform and refined photo-resist pattern on the wafer. A detection system has been proposed in which contaminants of the fluid medium between the lens and the wafer would be measured directly. See, e.g., U.S. Pat. No. 7,006,209. Unfortunately, such systems may not be usable with some simultaneous real-time photolithography processes because of interference between the detectors light output, the main light source, and local reflections. In addition, the single pass refraction of the detecting light through the liquid may result in a coarse measurement. Further, the system fails to account for the non-uniform distribution of the contaminants, at least the heavier of which may be more concentrated near their source at the boundary between the wafer and the liquid medium.